The present invention relates to a photoelectric conversion device and an image sensor and, more particularly, to reduction of fixed pattern noise (FPN) in a photoelectric conversion device having a plurality of photodetectors and peripheral circuits, configured with MOS transistors, integrally formed on a semiconductor substrate, and to an image sensor.
Recently, a one- or two-dimensional photoelectric conversion device, having a plurality of photodetectors and peripheral circuits for processing and controlling signals, integrally formed on a semiconductor substrate, has been developed. For example, a photoelectric conversion device having an internal reference voltage generator configured with an operational amplifier is proposed in Japanese Patent Application Laid-Open No. 9-65215.
An example of a circuit configuration of a photoelectric conversion device having a plurality of photodetectors and peripheral circuits integrally formed on a semiconductor substrate is shown in FIG. 7. In FIG. 7, peripheral circuits include a CMOS operational amplifier 40, and reference numeral 20 denotes a plurality of photodetectors; 22, a connecting pad; 30, a charge-voltage converters; 34, a switch; 35, a common output line; and 36, a shift register. The photodetectors 20 are connected to the charge-voltage converters 30 where charges generated depending upon incident light on the photodetectors 20 are converted to voltage signals. The voltage signals are sequentially outputted onto the common output line 35 in accordance with signals provided from the shift register 36 to the charge-voltage converters 30. The common output line 35 is connected to a positive terminal of the CMOS operational amplifier 40, and, after the input voltage signals on the common output line 35 are processed with impedance transformation by the CMOS operational amplifier 40, the voltage signals are outputted via the switch 34 and the pad 22.
The CMOS operational amplifier 40 includes a differential section 50 and an output section 51 as shown in FIG. 8 which is based on "Analog MOS Integrated Circuits for Signal Processing", R. Gregorian, G. C. Temes, pp. 170, FIG. 4.59. Generally, larger current flows in the output section 51 than other block in order to drive an external load. As for the circuit configuration of the output section 51, a source follower using an n-channel MOS (nMOS) transistor, whose mutual conductance g.sub.m is larger than that of a p-channel MOS (pMOS) transistor, an inversion amplifier and a push-pull circuit which are formed by combining nMOS and pMOS transistors are used.
In a MOS transistor, when a voltage is applied across a drain and a source while a channel is formed by applying a voltage to a gate, electric field becomes strong in the vicinity of the drain-side edge of the channel, which sometimes generates new electron-hole pairs due to impact ionization. Most of the carrier generated due to the impact ionization becomes substrate current and absorbed by a reference potential of the semiconductor substrate, however, a part of the carrier recombines. The recombination is accompanied by light emission, and the emitted light further generates new electron-hole pairs in the semiconductor substrate. The carrier generated in this manner becomes stray carrier which diffuses over the semiconductor substrate. When the stray carrier enters the photodetectors, ghost signals are generated in addition to essential signals generated in proportion to incident light. These ghost signals are a primary factor of fixed pattern noise in a photoelectric conversion device.
The measurement result, by the applicants of the present invention, of substrate current and drain current with respect to gate voltage Vg of nMOS and pMOS transistors is shown in FIG. 9. In FIG. 9, an abscissa shows the absolute value of the gate voltage, and an ordinate shows substrate current and drain current. The substrate current flowing in the nMOS transistor is about 10.sup.4 to 10.sup.5 larger than that in the pMOS transistor, which indicates that more electron-hole pairs are generated due to impact ionization in the nMOS transistor the in the pMOS transistor. Thus, since the fact that more substrate current flows in the nMOS transistor than in the pMOS transistor, stray carrier is more easily generated in a semiconductor substrate of an nMOS transistor than a pMOS transistor.
Further, substrate current in a MOS transistor depends upon the drain-source voltage more than the gate voltage. Experimental results show that the substrate current increases logarithmically with respect to increase in the drain-source voltage. Accordingly, it is determined that generation of stray carrier can be reduced by lowering the drain-source voltage.
Fixed pattern noise, with respect to a signal level, caused by stray carrier entering a plurality of photodetectors is not ignorable as sensitivity of the photodetectors improves.
FIG. 10B is a graph showing generated fixed pattern noise in a conventional one-dimensional image sensor (FIG. 10A) including the operational amplifier 40 using an nMOS transistor in an output section for driving a load. As shown in FIG. 10A, the photodetectors 20 and peripheral circuits 21 including the operational amplifier 40 having an nMOS transistor are formed on a semiconductor substrate 100. With this configuration, as shown in FIG. 10B, dark current corresponding to the portion where the output section is laid out is larger than other portions, which shows that fixed pattern noise is caused by stray carrier entering the photodetectors 20.